In situ or remote detection and monitoring of trace amounts of halogen-containing compounds in real-time is critical to the safety of humans, wildlife, and the environment. Volatile halogenated organic compounds, particularly those containing chlorine (Cl) or bromine (Br), are of considerable environmental importance due to their stratospheric ozone depletion properties, their contributions to the global greenhouse effect and their utility as indicators of biological inputs in biogeochemical studies of marine and atmospheric systems. There are also developing concerns that chlorinated organics cause cancers in adults and adverse health and reproductive effects in the offspring of both humans and wildlife. As a result, the development of monitoring methods for halogenated species has received increasing attention.
The most common method used to determine halogenated species under ambient conditions is cryogenic preconcentration of the sample followed by gas chromatography with electron capture detection (GC-ECD). The method is very sensitive, part-per-trillion (ppt) in level, with absolute limits of detection of the halogen atom on the order of 1-100 pg, depending on the compound. Despite high sensitivity, the GC-ECD technique requires extensive sample manipulation and a long measurement cycle, typically about one hour. Other methods of detection include mass and ion mobility spectrometry. Although these methods are also sensitive, they are not conducive to real-time, in situ measurements since the sample needs to be retrieved into an analysis chamber. In addition, acquisition and analysis is relatively slow and the prior-art apparatus is not readily portable due to its size.
Laser-based methods offer the advantages of high sensitivity and speed for the real-time monitoring of small species such as atoms, diatomics, and triatomics. These methods include laser-induced multiphoton ionization and laser-induced fluorescence. These methods currently enjoy limited applications. Many halogen containing compounds are large and lack distinguishing spectral features or absences of any features in the ultraviolet-to-visible (UV-Vis) spectral region. In addition, dissociation processes of the target molecules subsequent to laser excitation competes with ionization or fluorescence processes, causing a decrease or absence of ionization or fluorescence detection signal. As a result of these limitations, other approaches have been sought to overcome these obstacles.
One successful approach involves the use of the laser photofragmentation/fragment detection spectrometry. Selected references include: (1) U.S. Pat. No. 5,364,795, issued to Sausa et al. on Nov. 15, 1994; (2) Simeonsson, Lemire, and Sausa, "Trace Detection of Nitrocompounds by ArF Laser Photofragmentation/Ionization Spectrometry," Applied Spectroscopy, Vol. 47, No. 11, p. 1907, 1993; (3) Simeonsson, Lemire, and Sausa, "Laser-Induced Photofragmentation/Photoionization Spectrometry: A Sensitive Method for Detecting Ambient Oxides of Nitrogen," Analytical Chemistry, Vol. 66, No. 14, p. 2272, 1994; (4) Simeonsson and Sausa, "Trace Detection of Ambient Brominated Compounds by Laser-Induced Photofragmentation Detection Spectrometry," Spectrochimica Acta, Vol. 49B, p. 1545, 1994; (5) Arepalli, Presser, Robie, and Gordon, "Detection of Cl Atoms and HCl molecules by Resonance-Enhanced Multiphoton Ionization," Chem. Phys. Lett., Vol. 118, p. 88, 1985; and (6) Arepalli, Presser, Robie, and Gordon, "Detection of Br atoms by REMPI," Chem. Phys. Lett., Vol. 117, p. 64, 1985; all of which are incorporated herein by reference.
In the laser photofragmentation/fragment detection approach, the target molecule is photolyzed to produce characteristic fragments, such as atoms and small molecules which can be detected by multiphoton ionization (PF/MPI) or laser-induced fluorescence (PF/LIF) since they possess favorable combination of usually strong optical transitions and sharp, well-resolved spectral features. However, PF/MPI is susceptible to high background noise as well as a high degree of interference due to nonresonant multiphoton ionization. For LIF, laser scatter can present a significant problem. Moreover, the collection of total LIF from the photofragment is difficult since it is isotropic and incoherent.